Wavelength calibration tuning

The first and/or second dye lasers were tuned to the specific wavelength(s) to populate the desired level(s). The final laser in the excitation sequence (either the second or third laser) was then continuously scanned to obtain the Rydberg or autoionization spectrum. The spectrum and wavelength calibrations were recorded simultaneously on a two pen recorder. Wavelength calibration was obtained by directing a portion of the scan laser radiation to a monochromator that was preset at known U or Th emission lines from an electrodless lamp. 

Let us assume that Ai is known from calibration with a spectral line. When the laser wavelength is tuned, the next coincidence appears at A2 = Ai -h A A where... 

The temperature tuning coefficient of TeOa is 0.025 nm/°C, so it is clear that in order to combat the effects of thermal heating of the crystal due to incident optical and aconstic power, as well as environmental considerations, a good level of thermal control is needed. This can be in the form of a thermo-electrically cooled enclosure. This will deal to a certain extent with the need to achieve analyzer wavelength repeatability. However, it does not address the issue of wavelength reproducibility between AOTF modnles, which impacts the transportability of calibrations and datasets between analyzers. 

If a small fraction of the laser output is sent through a long Fabry-Perot interferometer with a separation d of the mirrors (Fig. 1.1b), the photodetector PD3 receives intensity peaks each time the laser frequency is tuned to a transmission maximum at v = mcjd (Vol. 1, Sects. 4.2-4.4). These peaks serve as accurate wavelength markers, which allow one to calibrate the separation of adjacent absorption lines. With a = 1 m the frequency separation Avp between successive transmission peaks is Avp = cjld = 150 MHz, corresponding to a wavelength separation of 10 nm at A. = 550 nm. With a semiconfocal FPI the free spectral range is cfM, which gives Av = 75 MHz for d = 0.5 m. 

The grating should be tuned to the zero order and the zero point adjusted to ensure agreement. The grating should be tuned to the laser wavelength and the software calibration adjusted accordingly (in cases where the laser line is fixed rather than tuneable, this step is performed upon installation). 

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